Dosimetric aspects of a national survey of diagnostic and interventional radiology in Switzerland

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1 Dosimetric aspects of a national survey of diagnostic and interventional radiology in Switzerland A. Aroua a) and I. Decka Institut Universitaire de Radiophysique Appliquée, Lausanne B. Burnand and J.-P. Vader Institut Universitaire de Médecine Sociale et Préventive, Lausanne J.-F. Valley Institut Universitaire de Radiophysique Appliquée, Lausanne Received 14 November 2001; accepted for publication 12 July 2002; published 12 September 2002 The effective dose delivered to the patient was determined, by modeling, for 257 types of examinations covering the different modalities of diagnostic and interventional radiology. The basic operational dosimetric quantities considered were obtained from the parameters of the examinations on the basis of dosimetric models. These models required a precise characterization of each examination. The operational dosimetric quantities were converted into doses to organs and effective doses using appropriate conversion factors. The determination of the collective effective dose to the Swiss population requires a number of corrections to account for the variability of several parameters: sensitivity of the detection system, age, gender, and build of the patient. The use of various dosimetric models is illustrated in this paper for a limited number of examination types covering the different radiological modalities, for which the established typical effective doses are given. With regard to individual doses, the study indicated that the average effective doses per type of examination can be classified into three levels: a the weakly irradiating examinations less than 0.1 msv, which represent 78% of the examinations and 4% of the collective dose, b the moderately irradiating examinations between 0.1 msv and 10 msv, which represent 21% of the examinations and 72% of the collective dose, c the strongly irradiating examinations more than 10 msv, which represent 1% of the examinations and 24% of the collective dose American Association of Physicists in Medicine. DOI: / Key words: diagnostic radiology, interventional procedures, patient dosimetry, dosimetric modeling, radiation protection I. INTRODUCTION The average dose to the population from medical exposure is estimated to be about msv/year in industrialized countries. 1 It comes mainly from diagnostic and interventional radiology and represents the highest contribution of man-made irradiation. It is therefore useful to determine periodically the collective dose to the population due to the medical application of x rays, since this collective dose is a function of various variable parameters. These parameters include a the demand for health care, b the access to health care, c the ageing of the population, d the expansion of certain diagnostic x-ray modalities and procedures and the decline of others, e the development of alternative diagnostic options, f the increase in the sensitivities of screen film combinations, g the trend towards digital radiology, and h the introduction of controls on quality required by new radiation protection regulations. 2 5 The determination of the collective dose due to diagnostic and interventional radiology requires the investigation of two categories of data: the frequency of the different types of examinations, on the one hand, and the dose delivered to the patient for each type of examination, on the other. The collective dose is then evaluated by convolving these two sets of data. The determination of the frequency of examinations is often made by nationwide or regionwide surveys covering a well-defined, often stratified, sample of hospitals and practitioners. The national trends are established from the sampled data by appropriate extrapolation, as was the case of the Swiss 1998 survey. 6 Dose to the patient is usually determined through studies of clinical examinations and radiological units. The quantities measured are, for example, the entrance surface kerma ESK in radiography, i.e., the air kerma free-in-air at the x-ray entry-surface plane of the patient, the kerma-area product KAP in fluoroscopy, and the dose-length product DLP in computed tomography CT. These dose quantities are then converted into organ equivalent doses and effective dose using appropriate conversion factors. In the United Kingdom the first British dose survey goes back to It covered 20 hospitals and dealt with 10 types of examinations. Concerning CT a campaign of dosimetric measurements was carried out between 1987 and covering 75 scanners. A national center for the registry of doses was set up in the early 1990s, the dosimetric measurements being classified according to a national protocol. 9 In Germany a 2247 Med. Phys , October Õ2002Õ29 10 Õ2247Õ13Õ$ Am. Assoc. Phys. Med. 2247

2 2248 Aroua et al.: Dosimetric aspects of a national survey 2248 survey on doses due to diagnostic radiology was undertaken in the early 1990s. 10 This survey focused on 30 types of examinations and 5000 measurements of KAPs were made in eight hospitals. In Norway three dosimetric surveys covered the period and focused on 68 types of conventional radiological examinations. 11 The recording of 5000 KAPs was carried out in about 50 hospitals. In the United States a large program monitoring trends in diagnostic radiology was launched in the early 1970s. The nationwide evaluation of x-ray trends NEXT 12,13 evaluates one or two kinds of examinations or one modality every year. The last published 14 NEXT data cover chest radiography, surveyed in NEXT surveys in the years 1995 through 2001 have been done for abdominal radiography, upper gastrointestinal fluoroscopy, paediatric chest radiography, dental radiography, computed tomography, and adult chest radiography once again. In Switzerland the dosimetric survey conducted in covered 500 radiological units and 20 types of examinations. These surveys resulted in large distributions of the dosimetric quantities. In fact the dose quantity for a given radiological examination is a function of a number of variables. They can be classified into three broad categories associated with a the patient, b the radiological technique, and c the radiological system. All these variables contribute to a different extent to the overall variability of the effective dose. 16 In this work another approach for establishing the dose to the patient associated with a given type of x-ray examination was adopted consisting in dosimetric modeling. In this case the dose quantities are not measured but computed, based on the technical parameters of the examination for typical patient characteristics, using appropriate formalisms. This methodology provides a practical tool for dose determination. It allows establishing the dose to the patient as a function of the basic parameters of the examination, while accounting for any variation in these parameters. The present work was carried out within the framework of a nation-wide survey that aimed at determining the radiation doses delivered in Switzerland in 1998 by the various radiological examinations performed in diagnostic and interventional radiology. This paper deals exclusively with the dose investigation. The results of the frequency study are presented in the full report. 6 Moreover, although the nation-wide dosimetric study dealt with 257 types of examinations, the dosimetric models are illustrated in this paper by a limited number of examinations. II. MATERIAL AND METHODS evaluation of an operational dose quantity which can be ESK, DLP or other similar quantity. The operational dose quantity is then converted into organ equivalent doses and effective dose using appropriate conversion factors. The effective dose is hence expressed as E ESK e ESK for radiography and fluoroscopy, 1 E DLP e DLP for computed tomography, where e ESK and e DLP are factors that convert, respectively, ESK and DLP into E. B. Operational dose quantity calculation 1. Classification of the examinations The operational dose quantity for a given type of examination is calculated based on the technical parameters of the equipment and the radiological procedure related to this type of examination. All examinations encountered in diagnostic and interventional radiology in Switzerland were classified into 257 types covering different categories according to the radiological technique used and to the aim of the examination: diagnostic or interventional. These categories include a examinations involving plane film radiography 54 types, b examinations involving plane film radiography and fluoroscopy 33 types, c angiography diagnostic examinations 35 types, d interventional examinations 43 types, computed tomography CT examinations 47 types, mammography 2 types, bone densitometry examinations 4 types, conventional tomography examinations 6 types, and dental examinations 33 types. Ten physicians from different medical specialties and the technicians of the Lausanne University Hospital CHUV were then asked to provide an accurate typical definition for each of the 257 types of x-ray examinations: number of images, fluoroscopy time, number of CT slices, etc., in addition to the technical parameters used. This resulted in a technical document that was then sent for validation to the technicians in medical radiology at the eight major Swiss hospitals. The process of validation consisted in comparing for each type of examination the data stated in the document with that used in the routine work at their own hospital and to report any difference. The comments collected from the eight technicians were analyzed and a typical technique was established for each type of examination with parameters corresponding to the average values at the national level. The parameters for the full set of 257 types of examinations are presented in the full report available on the Internet. 6 A. Calculation of the effective dose The fundamental quantities used for determining the collective dose to the population are the organ equivalent doses, H T, and the effective dose, E as defined by the ICRP. 17 The determination of the organ equivalent doses and the effective dose is a two-step process. It consists first in the 2. Calculation formalism In the following, the formalisms used to establish the operational dose quantity for a few types of examinations, performed at typical conditions, are given. a. Radiography. In the case of a radiographic projection, the ESK takes this empirically based form

3 2249 Aroua et al.: Dosimetric aspects of a national survey 2249 TABLE I. Typical technical parameters for chest radiography. View TABLE II. Typical technical parameters for intravenous urography. Part of the body Technical parameter AP PA Lateral Technical parameter Abdomen Pelvis Frequency % Tube voltage kvp Tube charge mas Filtration mm Al Focus-to-film distance cm Field size at the skin entrance plane cm cm Sensitivity a Grid a yes yes yes a Not used in the calculation, reported for documentation only. ESK mgy) K mgy m 2 /mas U kv F A mm Q mas 1 FSD m ] 2, where U is the tube voltage, Q is the tube charge, F A is the filtration expressed in mm of aluminum, and FSD is the focus-to-skin distance. K is an empirically determined constant specifying the radiological unit. This formalism, we have been using for many years, was established based on the empirical data given in a Scientific Report by the Hospital Physicist Association 18 and was validated several times by measurements. The range of values of K reported in the literature 15,18 20 was studied in order to see what suits best the types of x-ray units mostly used in Switzerland and a typical value of 0.1 mgy m 2 /mas was finally adopted. A radiography examination is generally composed of one or several projections. For example, in the case of chest radiography the way this examination is performed was investigated. This varies from place to place and typical chest radiography was defined as consisting of a PA view in the upright position 75% of the cases or a supine AP view 25% of the cases. Additionally a lateral view was considered in 25% of the cases. This additional view is taken for erect patients exclusively. The typical technical parameters for this type of examination, as established from the data gathered from the eight major Swiss hospitals, are shown in Table I. b. Fluoroscopy. This category of examinations involves fluoroscopy in addition to radiography, and both modalities must be considered in dose calculation. In the case of a fluoroscopy, the ESK was also used as input data for the calculation program. It takes the same form as above, the charge being now replaced by the current I in ma units times the exposure time t in seconds. The typical technical parameters for an intravenous urography IVU examination, which involves the exposure of two parts of the body abdomen and pelvis, as established from the data gathered from the eight major Swiss hospitals, are shown in Table II. 2 Fluoroscopy Duration s Tube voltage kvp Tube current ma 3 3 Focus-to-image intensifier distance cm Diameter of the image intensifier inches 9 9 Radiography Number of images 5 2 Filtration mm Al 3 3 Tube voltage kvp Tube charge mas Focus-to-film distance cm Field size cm cm Sensitivity a Grid a yes yes a Not used in the calculation, reported for documentation only. c. Angiography and interventional radiology. In this category of examinations, in addition to fluoroscopy a relatively large number of digital images are acquired. In the dose calculation the fluoroscopy process is handled separately from the procedure of registering a series of images. The typical technical parameters for an angiography examination coronary angiography CA and a typical interventional radiology examination percutaneous Transluminal Coronary angioplasty PTCA, as established from the data gathered from the eight major Swiss hospitals, are shown in Table III. d. Computed tomography. In CT dosimetry, the computed tomography dose index CTDI is defined as the integral along a line parallel to the axis of rotation of the dose profile for a single rotation and a fixed table position, divided by the nominal thickness of the x-ray beam. It is expressed in terms of absorbed dose to air in mgy. The CTDI in air is converted by the dose calculation program see below into a weighted computed tomography dose index, 21,22 CTDI w, representing TABLE III. Typical technical parameters for CA and PTCA examinations. Technical parameter CA PTCA Cine mode Tube voltage kvp Tube current ma Focus-to-image intensifier distance cm Number of sequences 10 7 Part of the body thorax thorax Total number of images Exposure time per image ms 5 5 Total effective exposure time s Fluoroscopy Tube voltage kvp Tube current ma Duration min 4 10 Diameter of the image intensifier inch 9 9 Field size cm

4 2250 Aroua et al.: Dosimetric aspects of a national survey 2250 TABLE IV. Typical technical parameters for CT of the abdomen. Technical parameter Series 1 Series 2 Number of scans 1 1 Plane of first slice hepatic dome iliac crest Plane of last slice iliac crest public symphysis Mode helical axial Slice thickness mm 8 10 Slice spacing mm 1 0 Pitch overlap Length of scanned volume mm Tube voltage kvp Tube current ma Rotation time s CTDI w mgy the average dose that would be absorbed by the central slice within a 100 mm range of contiguous scanning, and defined as follows: 21,22 CTDI w mgy 1/3 CTDI c mgy 2/3 CTDI p mgy, 3 where CTDI c is measured at the center of a homogeneous cylinder of polymethylmethacrylate PMMA, with diameters of 16 cm head or 32 cm body, and CTDI p is measured 10 mm below the surface of the phantom, and represents an average of measurements at four different locations around the periphery of that phantom. The dose-length product DLP is defined as follows: 21,22 DLP mgy cm) CTDI w mgy t cm n, where t relates to the slice thickness, n relates to the number of slices. The typical technical parameters for the CT examination of the abdomen that consists of two series of CT slices one scan each, as established from the data gathered from the eight major Swiss hospitals, are shown in Table IV. C. Conversion of the operational dose quantity into effective dose 1. Methods for the determination of the conversion factors As stated above, the dosimetric conversion factors are coefficients, which enable transforming the operational dose quantities into equivalent doses to organs and tissues or effective dose. The dosimetric conversion factors are usually determined for typical conditions and established a Experimentally, by measuring often using thermoluminescent dosimeters TLD on a physical anthropomorphic phantom the doses deposited into the organs of interest; 15,23 29 b Numerically, using a Monte Carlo calculation method and either a mathematical or a voxel phantom. The energy deposited into the volume of each organ of interest is recorded after each interaction of the radiation with the material of the organ; c Semiempirically, by a computation using an analytical method and an anatomic tomographic phantom. The 4 profiles of the dose distribution in the three directions are used to calculate the doses to the organs of interest Conversion factors for radiography and fluoroscopy The package ODS-60 Organ Doses Calculation Software is used for dosimetric calculations in the case of radiographic and fluoroscopic examinations, that is for the conversion of ESKs into doses to various organs and tissues. The ODS-60 package, based on conversion factors using the semiempirical method, was developed by the Finnish Center for Radiation and Nuclear Safety STUK in collaboration with the Institute of Radiology of the Russian Ministry of Public Health Central Scientific Institute of Radiology and Röntgenology of the Ministry of Health, St. Petersburg and the Institutes of Physics and Biomedical Sciences of the University of Helsinki. Compared to other calculation methods which use mathematical phantoms, this software program has the advantage of using a gender and build adjustable phantom and enabling the precise introduction of the irradiation geometry. A description of the phantom and the adjustment algorithms is available in one of STUK s detailed reports. 64 There have been comparisons of the results obtained using ODS-60 with those of other methods, such the NRPB 65 and GSF 66 calculations, or with experimental results. 67 This work showed that the ODS-60 package yields values larger than those of Monte Carlo method based programs, by 7% to 33% according to the developers of the ODS-60. According to a German study 66 the difference between the ODS data and the GSF data may reach a factor of 4.4 and 5.0 in the particular case of the bladder examination for a woman and a man, respectively. But this is an extreme value, corresponding to a case where the delimitation of irradiated organ is very different with the two different phantoms used, is not representative of the general situation. These discrepancies have been attributed mainly to the differences between the anatomic phantom used in the ODS-60 and the mathematical phantom used in the Monte Carlo method. The location of organs is appreciably different and their form is more complex in the case of the anatomic phantom. The differences in the area of the field and the irradiation geometry are another source of discrepancy. The ODS-60 program is used in this work because it provides a the facility to vary the irradiation geometry and thus to cover a much larger number of examination types; b the flexibility to modulate the technical parameters of the examination and the parameters of the patient weight, height, and gender, a feature which is not available in other programs; c a faster calculation speed than similar programs. Regarding radiography, the technical parameters for each type of examination and each projection were used in the calculation of doses to organs by means of the ODS-60 program. For each projection, the irradiation geometry, voltage, focus-to-patient distance, filtration, field area and ESK are

5 2251 Aroua et al.: Dosimetric aspects of a national survey 2251 given as input data, which also specify the height, weight, and gender of the patient. As the ODS-60 accepts the ESK rather than the charge as input datum the former is calculated beforehand using the above-mentioned formulas. The ODS-60 package was used for all the examinations dealing with the head and neck, the thoracic and abdominal regions, and the pelvis. The calculations for men and women are run separately. On the basis of ICRP data, 68 the average values of 58 kg, 160 cm and 70 kg, 170 cm were used for the weight, height of women and men, respectively. For each of the examinations involving radiography and fluoroscopy conventional, angiographic, and interventional, the corresponding ESK was calculated for the radiography and the fluoroscopy separately. The ODS-60 package was then used for each of the two contributions by taking especially into account the area of the field corresponding to the radiography and the fluoroscopy. The doses to organs were added at the end of the calculation. 3. Conversion factors for computed tomography In the case of CT examinations, the program CT DOSE 69 is used to convert the operational dose quantity into various organs and tissues for the different protocols used. This program was developed by the Danish National Institute of Radiation Hygiene, in Herlev, in collaboration with the Department of Biomedical Engineering of Aarhus University in Denmark. It is based on the use of a mathematical phantom and the Monte Carlo calculation method. 53 The CT DOSE data base covers about 60 scanner types and settings. The program takes as input data the type of scanner used, the CTDI, the voltage, the filtration, the current, the scanned volume, the scanning time, the slice thickness as well as the distance between slices. Therefore the CT DOSE program does not convert directly the DLP into organ doses and effective dose. It converts rather the CTDI into organ doses and effective dose associated to each slice composing the CT scan, and performs a summation at the end. TABLE V. Weight percentages of radiation-sensitive organs in the irradiated field. Examination Bone marrow Skin Bone surface Muscle Arm Elbow Forearm Wrist Hand Finger Leg Knee Patella Foreleg Ankle Calcaneum Foot Toe Special cases In some particular cases, when the ODS-60 and CT DOSE programs do not enable dosimetric evaluations, the doses to organs are determined alternatively. With regard to the extremities, the doses to organs were calculated by assuming that the dose was uniform throughout the extremity for radiography and fluoroscopy examinations. Only the bone marrow, the skin, the bone surface and the muscles were considered. Since the muscle is part of remainder organs or tissues, it was considered only when its associated equivalent dose was larger than the largest one of the others. To determine the equivalent dose to each of the four organs considered, the entrance dose was multiplied by the percentage of the organ in the irradiated field. The weight percentages presented in Table V for some types of examinations were estimated using the data from Publication 23 of ICRP. 68 Similarly, the CT DOSE package does not take into account the upper limbs, which had to be computed under the assumption of uniform dose throughout the extremity. This is also the case for the examinations of the ankle, the foot, the calcaneum, and the leg. For the following dental x-ray examinations: apical and status, bitewing, occlusal, orthopantomography OPG and CT, the dose factors determined recently by Dula and Mini 70,71 were adopted after a prior comparison with the data in the literature For apical examinations, the dose factors were given according to the tooth position: jawbone or mandible, molar, premolar, canine or incisor. Therefore the conversion factors for apical examinations were determined by weighting the factors specific to the tooth position by the proportion of each tooth position to the total number of apical examinations data gathered in the Swiss 1998 survey 6 was used for this purpose. The dose factors are also given for round and rectangular collimators. Average dose factors were calculated by weighting the fractions of round and rectangular collimators data from the Swiss 1998 survey 6. For mammography, the average glandular equivalent dose corresponding to both breasts was considered. It is calculated by multiplying the entry dose by a mean conversion factor estimated here at 0.2 msv/mgy. 76 With regard to bone densitometry, the effective doses were taken from recent literature These are associated with a bone densitometer of QDR2000 type, based on dual x-ray absorptiometry DXA and manufactured by the firm Hologic. D. Error on the effective dose The error on the typical effective dose for a given type of examination stems essentially from a the error on the definition of the examination type, that is to say on the protocol used as typical and the technical parameters chosen; b the error associated with the algorithm for calculating doses to organs, i.e., calculation errors of the ODS-60 and CT DOSE packages in our case. A study of the accuracy of dose indications in diagnostic radiology 16 has been made. This study involved various types of variables, relevant to dosimetric

6 2252 Aroua et al.: Dosimetric aspects of a national survey 2252 calculations, which relate to the patient, the radiological technique, and the radiological system. The study showed that the relative error on the typical effective dose is in our case of the order of 15%. E. Corrections applied to establish the collective effective dose Since operational dose quantities and conversion factors are established for average characteristics of individuals, radiological systems, and techniques, they are used to infer reference values of the effective dose for each type of examination. When calculating the collective effective dose man Sv associated with a particular type of examination, several corrections may be required to account for possible bias in the reference values. The collective effective dose associated with a given type of examination can be expressed using the following formalism: E coll NEC p C t C s, where N relates to the total number of examinations of a given type performed within the population; E is the reference value of the effective dose for that given type of examination, inferred from Eqs. 1 4, from average values of the system techniques, from typical values of system radiation output, and from dose conversion factors calculated with the computer program ODS-60 for radiography and fluoroscopy or CT DOSE for computed tomography ; C p is a factor correcting both the operational dose quantity and the conversion factor for the variability of the patient build; C t is a factor correcting both the operational dose quantity and the conversion factor for the variability of the technical parameters of the examination; C s is a factor correcting the operational dose quantity for the variability of the radiation output of the radiological system and the system detection sensitivity. 1. Correction for the variability of patient build and gender a. Adult patient. A study of the distribution of weights and heights according to age of the Swiss population was made in order to deal with the effect of the build. The data obtained by two Swiss surveys Monica-1992 and OFS-1997 was used. 81 The aim of this investigation was to check if the ESK calculated for typical build is comparable to a the ESK accounting for interindividual build variability within a given age category, and b the ESK accounting for build variability with patient age. For each age group two quantities were calculated. 1 A standard entrance surface kerma, ESK s, corresponding to the mean value of the weight distribution for the age group considered. 2 A weighted entrance surface kerma, ESK w, which is a weighted sum of the ESK corresponding to the various weight groups within the age group considered. Table VI shows the ratio ESK s /ESK w for two types of examinations corresponding to the age group years 5 TABLE VI. Illustrative example for the weight effect. The calculations are made for the age group years. Examination Gender ESK s /ESK w Abdomen AP Man Woman Lumbar spine Lateral Man Woman with a weight variability within the interval kg. It appeared from the results obtained for the various age groups that within a given age group, the typical value of ESK corresponding to the mean build for the age group is comparable within about 5% with the weighted ESK accounting for the weight distribution within this age group. The calculation model used here is crude and has only a control function. Therefore no correction was introduced to account for interindividual variability. Concerning the age-related build variability, for each of the 10 age groups considered covering the range years, the ESK corresponding to the mean build characteristics was calculated and compared to the typical ESK used in the dosimetric model. The detailed results of these calculations are presented in Appendix 10 of the full report. 6 The study showed that the maximum deviation of the ESK calculated for all age groups relative to the typical value of ESK is of the order of 12% for woman and 18% for man 15% on average. However, it is thought that the mean deviations are lower about 5% due to interindividual variability and therefore no correction for age was applied. In the dosimetric model the technical characterization of the examinations was made for a typical patient with no man woman differentiation. The extent to which the ESK is affected if the patient characteristics were man and woman specific was investigated for a number of radiographic examinations including abdomen, pelvis, and lumbar spine, where the build variability with gender is significant. By considering the same exit dose for both genders, the mean deviation of the man and woman specific ESK from the typical nongender specific value was found to be of the order of 30%. A gender correction factor of 1.3 for man and 0.7 for woman was therefore established. This correction has little effect in the computation of the collective effective dose since for the huge majority of types of examinations the total number of examinations is equally distributed between both genders. b. Paediatrics. In the case of children, an age-dependent correction model for the operational dose quantity was established on the basis of the data available in the literature. The depth dose percentage DDP method was used. The DDP values measured in a water phantom at the depths corresponding to the various thicknesses of child and adult patients for typical diagnostic x-ray spectra were taken from the literature. 82,83 The ratio of ESK for a child to ESK for the adult can be expressed as follows:

7 2253 Aroua et al.: Dosimetric aspects of a national survey 2253 TABLE VII. Factors for correcting the dosimetric quantities for age in paediatrics for radiography, fluoroscopy, and computed tomography. Age years Adult Mean height cm Mean weight kg Mean diameter cm Radiography and fluoroscopy a ESK child /ESK adult b e ESK child /e ESK adult ESK e ESK child / ESK e ESK adult Computed tomography c DLP child /DLP adult d e DLP child /e DLP adult DLP e DLP child / DLP e DLP adult a Based on data from Refs. 82 and 83. b Based on data from Ref. 86. c Based on data from Ref. 85. d Based on data from Ref. 22. ESK child ESK adult DDP adult DDP child. 6 TABLE VIII. Distribution of sensitivities for lumbar spine radiography. Establishment category Number of screenfilm combinations f 100 % f 200 % f 400 % f 800 % f unknown % Chiropractic General medicine Rheumatology All practitioners Hospitals All categories Table VII shows the ratio ESK child /ESK adult for different age groups. These are mean values averaged over abdomen, thorax, and skull examinations. They were applied to all radiography and fluoroscopy examinations. The results obtained with the DDP method were validated by comparison with age-specific ESK from the literature. 84 Concerning CT examinations, the factors used to correct the DLP for the age are established from the work of Ware et al. 85 They are given in Table VII as well. The factors converting the operational dose quantities into organ equivalent doses and effective dose have also been corrected for age. The data published by NRPB 86 was used to this effect. Average values calculated over the 10 types of examinations considered in the NRPB work, are presented in Table VII. They are used for all radiography and fluoroscopy examinations. Concerning the CT examinations, similar correction factors see Table VII were established from the work of Nagel. 22 The data in Table VII is used to correct the effective dose in the case of paediatric examinations. The ratios ESK child /ESK adult and DLP child /DLP adult increase with age, due to the increase the ESK or DLP with increasing build of the patient. On the other hand, the ratios e ESK child /e ESK adult and e DLP child /e DLP adult decrease with age due to the increasing distances between the irradiation field and the different body organs and tissues. The product of both ratios which represents E child /E adult tend to come closer to one by compensation. Table VII shows that for children this product is smaller than one in the case of radiography and greater than one for computed tomography. This reflects the fact that unlike the case of radiography where ESK is adapted to children automatic control in paediatric computed tomography the technical parameters used at the time of the survey were not sufficiently adapted to the build characteristics of children, resulting in relatively high values of DLP for them, but since then the radiology community is aware of this problem and an improvement is being made to better adapt the CT technical parameters in paediatrics Correction for the variability of the radiological technique The radiological technique is defined by the technical parameters of the examination: number of radiographic projections, duration of fluoroscopy, field size, tube filtration, focus-to-film distance, voltage, charge, number of CT slices, slice thickness, etc. The differences registered in the technical parameters for the 10 physicians of various medical specialties and the technicians of the main Swiss hospitals and their deviations from the typical values used were analyzed. This analysis showed that these deviations, although significant for a few types of examinations, do not affect the mean dose. Assuming that the variability of the data collected from the physicians and the main hospital all over the country is representative of the variability at the national level, which is difficult to establish accurately, it was decided to apply no correction for this type of variability. 3. Correction for the variability of the radiological system In the dosimetric model for radiography and fluoroscopy a typical value of the K constant see Eq. 2 was adopted. This value corresponds to a typical output of the radiological unit. In the absence of data concerning the variability of this output, data on the variability of the detection system can be used instead, to apply the necessary correction. This is justified by the fact that the output of the radiological unit and the sensitivity of the detection unit are inversely related, and a change in the output of the radiological unit is automatically reflected by an equivalent change in the sensitivity of the detection unit. The distributions of the sensitivities of the screen film combinations for the various types of radiographic examinations obtained by the nationwide survey were therefore used to correct for the variability of the radiological system. Table VIII shows some examples of such distributions for lumbar spine radiography. Since in the dosimetric model a typical value of the sensitivity was used for each type of examination based on the recommendations of the Swiss health authorities, a correction factor, F SENS, for each type of examination was calculated to account for the distribution of the screen film com-

8 2254 Aroua et al.: Dosimetric aspects of a national survey 2254 TABLE IX. Distribution of the CTDI n in mgy/mas, for the head and the body and two kvp values. Type of CT scanner Number of CT scanners 120 kvp head 120 kvp body 140 kvp head 140 kvp body Simens plus Simens CR Toshiba Xpress Toshiba TCT Toshiba Xvision Simens HiQ Simens Somaton plus Elscint Philips GE CT/I Toshiba XspeedII GE Picker PQ Siemens AR.HP Simens Somaton AR-C GE Prospeed GE Pace Sytec binations. For example, in the case where the typical sensitivity considered in the dosimetric model is S400 and the sensitivities S100, S200, S400, and S800 are used in practice, this factor takes the form F SENS 4 f S100 2 f S200 f S f S800, where f S100, f S200, f S400, and f S800 are the percentages of use of sensitivities S100, S200, S400, and S800, respectively. The linear form of F SENS is due to the fact that the sensitivity the speed S of the detection system is inversely proportional to the dose received and hence to the patient effective dose. For example, F SENS, calculated for all medical specialties and accounting for the distribution of sensitivities for each type of examination as established by the nationwide survey, equals 2.12 for cervical spine, 2.08 for thorax, 1.25 for thoracic spine, 1.80 for abdomen, 1.22 for lumbar spine, and 1.76 for pelvis. In the case of CT the output of the scanner is characterized by the normalized axial dose free in air (CTDI n ), 86 a quantity directly proportional to the normalized CTDI w, and required as input for dose calculation by the CT DOSE program. Table IX shows the distribution of CT scanners used in Switzerland, established by the nationwide survey. The associated CTDI n for the head and the body and for 120 and 140 kvp, taken from the literature, 22,69,90 is presented. Since a CTDI n of 0.2 mgy/mas (CTDI w of 14.6 mgy was used as the reference value CTDI ref in the initial dosimetric model validated by the technicians of several university hospitals, two correction factors, F CTDI, one for the head and the other for the body examinations were calculated to account for the distribution of the normalized CTDI. These factors are expressed as follows: F CTDI jn j CTDI j CTDI ref, 8 where N j is the fraction of the total number of scanners with normalized CTDI j. Here again the linearity of F CTDI comes 7 from the fact that the patient effective dose is directly proportional to the CTDI n. The F CTDI calculated for the head and the body examinations are 1.58 and 1.20, respectively. III. RESULTS AND DISCUSSION The organ equivalent doses and the effective dose to the patient were calculated for the 257 types of examinations. The full dosimetric results are given in the detailed report. 6 An example in each category already mentioned is presented in Table X for illustration. Using dosimetric modeling one can relate the patient effective dose to the technical data of the equipment and the parameters of the radiological procedure presented above. The dosimetric modeling required the full definition of 257 examination types; the main advantage of such a modeling is the possibility offered to recalculate easily and rapidly the patient effective dose to account for any change in the technical parameters. For instance, if a different protocol were used in coronary angiography, with 400 images instead of 500, 3 min of fluoroscopy instead of 4 min, and 85 kvp instead of 80 kvp in radiography, then a new effective dose to the patient can be easily computed using the formalism presented above. These changes would lead to 10% dose decrease in the radiography part, to 15% dose decrease in the fluoroscopy part, and to 11% dose decrease in total. Similarly if the CT examination of the abdomen is performed on a different CT scanner with a normalized axial dose free in air of 0.24 mgy/mas instead of 0.20 mgy/mas, and if a pitch of 1.5 instead of 1 is considered, the simple calculation of the new effective dose to the patient, using the above-mentioned formalism, would reveal a 35% decrease of this quantity. The effective doses presented in Table X were compared to the data reported in the UNSCEAR Report 2000, 1 as shown in Table XI. One can see that the effective doses established in this work are within the ranges of variation

9 2255 Aroua et al.: Dosimetric aspects of a national survey 2255 TABLE X. Examples illustrating the dosimetric modeling. Radiological modality Type of examination Mean ESK mgy Mean DLP mgy cm Mean effective dose msv Man Woman Radiography Chest Radiography & Intravenous fluoroscopy urography Angiography Coronography Interventional radiology PTCA CT Abdomen Dental radiology Apical Special modalities Mammography 2 breasts reported in the literature, which are relatively wide. Due to the absence of data on the conditions associated with these reported doses it is difficult to explain any differences. Table XII presents typical values of the effective dose, for both females and males, and this for the different radiological modalities. Table XII indicates also the ranges of variation of the effective dose within each radiological modality, i.e., the values of E corresponding to the least and the most irradiating types of examinations within the modality. In the case of radiography, for instance, the range of variation is about five orders of magnitude, which reflects the irradiation scale in radiography ranging from a very low dose for finger radiography to a relatively high dose for the radiography of the lumbar spine. The average effective doses must therefore be used with caution since they hide a large variability. However, in terms of average effective doses, the different radiological modalities can still be classified in the following increasing order of irradiation: dental, radiography, CT, radiography, and fluoroscopy, interventional. The 257 types of examinations were grouped in three dosimetric categories based on the dose ranges. The number of examination types in each category and its contribution to the total number of examinations and the collective dose, given in the full report, 6 are presented in Table XIII. Category 1: About 60 examinations giving an average effective dose lower than 0.1 msv to the patient. This group of examinations accounts for 78% of the total number of examinations and 4% of the collective dose. It includes for instance examinations of the extremities and the thorax, bone densitometry, and dental examinations. Even though the dose level in this category is relatively weak, the effort of reducing the doses must be maintained mainly by using sufficiently sen- TABLE XI. Comparison with some dosimetric data reported in the UNSCEAR Report The effective dose is given in msv. Country Chest IVU Coronography PTCA Finland Germany Japan Netherlands Norway Sweden United Kingdom Range of variation a Average a Switzerland this work Country CT abdomen Dental apical Mammography b Finland Germany Japan Netherlands Norway 12.8 Sweden United Kingdom Range of variation a Average a Switzerland this work a Chest, 18 countries; IVU, 14 countries; CT, 9 countries; Apical, 10 countries; mammography, 23 countries. b The effective dose corresponds to the product of an ESK of 10 mgy, a conversion factor ESK to glandular equivalent dose of 0.2 msv/mgy, a breast tissue weighting factor of 0.05, and two projections.

10 2256 Aroua et al.: Dosimetric aspects of a national survey 2256 TABLE XII. Effective doses msv for some typical examinations as well as ranges of variation for the various radiological modalities considered. The range shows the values of E corresponding to the least and the most irradiating types of examinations within the modality. Category Examination E man E woman E average Radiography Range of variation Lumbar spine Thoracic spine Abdomen Radiography Range of variation and fluoroscopy Small intestinal transit Cholangiography Pyelography Angiography Range of variation Pulmonary angio Renal angiography coronary angiography Interventional Range of variation TIPS Abdominal embolisation Coronary dilatation CT Range of variation Lumbar spine Abdomen Thorax Dental Range of variation Apical Bite-wing OPG Special Range of variation examinations Mammography 0.2 a 0.20 Tomography of pelvis Bone densitometry of whole body a This corresponds to an ESK of 10 mgy, a conversion factor ESK to glandular equivalent dose of 0.2 msv/mgy, a breast tissue weighting factor of 0.05, and two projections. sitive film screen combinations because of the large number of these examinations. Category 2: About 160 examinations giving an average effective dose ranging between 0.1 msv and 10 msv to the patient. This group of examinations accounts for 21% of the total number of examinations and 72% of the collective dose. It comprises for instance examinations of the head, spine, and abdomen and conventional fluoroscopy. This class of examinations involves average individual doses, which account for the largest contribution to the collective dose and thus are important in the dose reduction process. Category 3: About 30 examinations giving an average effective dose larger than 10 msv to the patient. This group of examinations accounts for 1% of the total number of examinations and 24% of the collective dose. It includes part of the CT examinations, angiographic ones, and those of interventional radiology. These examinations contribute to a larger individual irradiation than that suggested by the average effective dose to the population. Under conditions of extreme exposure in interventional radiology, some deterministic effects may arise mainly on the skin, in addition to stochastic effects. IV. CONCLUSION This work has enabled the determination of the effective doses associated with 257 types of examinations covering the various modalities in both diagnostic and interventional TABLE XIII. Characteristics of the three dosimetric categories. Dosimetric category Effective dose range Number of examination types Contribution to the total number of examinations % Contribution to the collective dose a % msv msv 10 msv msv a Collective dose 7100 man Sv.

11 2257 Aroua et al.: Dosimetric aspects of a national survey 2257 radiology. The dosimetric modeling was an adequate way to evaluate the patients doses based on the technical parameters of the equipment and the radiological procedure used in Switzerland in It represents a high-performance instrument allowing dose re-evaluation whenever the radiological technique changes, and hence the follow-up of the time evolution of the collective dose. This investigation allowed the assessment of the filmscreen combinations in radiography. The use of the recommended sensitivities is not the rule everywhere in Switzerland and many still use low-sensitivity film screen combinations. This has a direct significant incidence on the collective dose. For example, if the recommended sensitivities were used then the corresponding collective dose in radiography would have been 2000 man Sv instead of 3000 man Sv, i.e., a dose reduction by one-third. This study confirmed that the most dose-intensive modalities are those involving fluoroscopy, interventional radiology, and CT. The spread of CT multislice technique, the increase of interventional procedures and the advent of digital techniques will present new challenges in terms of dose. All these modalities deserve special attention and effort in order to keep the doses under control. ACKNOWLEDGMENTS This research project was funded by the Swiss Federal Office of Public Health. Members of The Radiation Physics Department of the Bern University Hospital, the Department of Diagnostic and Interventional Radiology of the Lausanne University Hospital, the main Swiss hospitals and private surgeries were made available for this project and their expertise on several dosimetric aspects is appreciated. A group of experts, made up of the representatives of the major Swiss medical societies, was in charge of following the progress of this work. 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